1. Field of the Invention
This invention relates to non-volatile semiconductor memory and particularly to circuits and methods for erasing and programming flash memory.
2. Description of Related Art
A conventional erasable non-volatile semiconductor memory cell includes a transistor, such as floating gate transistor, having a threshold voltage that can be changed to represent stored information. Typically, to write data in such memory cells, the memory cells are first erased to set the threshold voltage of the memory cells in an erased state, for example, a low (or a high) threshold voltage level, where the threshold voltage of the memory cells are known. Data are then written to the memory cells by programming selected cells.
Data may be written or stored in many forms. For example, data can be stored in a binary form with one bit per memory cell. Binary data storage leaves some memory cells in the erased state to represent a first bit value (zero or one) and programs the threshold voltage of other memory cells to a second state representing a second bit value (one or zero). Alternatively, data can be stored with multiple bits per memory cell by programming the threshold voltage of a memory cell to a level within a set of discrete states that represent the possible values that can be stored in the memory cell. Data can also be stored in an analog form with one analog value (or sample) per memory cell by programming the threshold voltage of each memory cell to a level within a continuous range of threshold voltages that is in one-to-one correspondence with the range of analog values being written.
Fowler-Nordhiem tunneling is a typical mechanism for an erase process in a conventional non-volatile memory such as an EEPROM, an EPROM, or a flash memory. Fowler-Nordhiem tunneling is a relatively slow process and thus reduces the write speed in memories that erase individual cells immediately before writing. Accordingly, flash memory, which simultaneously erases a block of memory cells, has been developed. However, erasing a block of memory cells in a flash memory may delay the start of writing data, and such delays can be a problem in a real time data recording system. For example, when a non-volatile memory records a continuous signal, part of the signal may be lost while an erase process prepares a block of memory cells for recording. A non-volatile semiconductor memory capable of immediately commencing and continuously maintaining data recording operations is sought.